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[TLI][PowerPC] Introduce TLI query to check if MULH is cheaper than MUL + SHIFT

Authored by amyk on May 23 2020, 2:46 PM.

Description

[TLI][PowerPC] Introduce TLI query to check if MULH is cheaper than MUL + SHIFT

This patch introduces a TargetLowering query, isMulhCheaperThanMulShift.

Currently in DAG Combine, it will transform mulhs/mulhu into a
wider multiply and a shift if the wide multiply is legal.

This TLI function is implemented on 64-bit PowerPC, as it is more desirable to
have multiply-high over multiply + shift for words and doublewords. Having
multiply-high can also aid in further transformations that can be done.

Differential Revision: https://reviews.llvm.org/D78271